The present invention relates to a vertical-heat-treatment apparatus for heat-treating a plurality of target substrates, such as semiconductor wafers, all together in a semiconductor processing system. The term "semiconductor processing" used herein includes various kinds of processes which are performed to manufacture a semiconductor device or a structure having wiring layers, electrodes, and the like to be connected to a semiconductor device, on a target substrate, such as a semiconductor wafer or an LCD substrate, by forming semiconductor layers, insulating layers, and conductive layers in predetermined patterns on the target substrate.
Vertical-heat-treatment apparatuses are known as apparatuses for subjecting a number of semiconductor wafers to a predetermined heat treatment all together. FIG. 9 is a cross-sectional view showing a vertical-heat-treatment apparatus 2 of related arts.
In the heat treatment apparatus 2, wafers W are vertically stacked with gaps therebetween on a wafer boat 42 of quartz, and then the boat is loaded into a vertical process chamber 8, so that the wafers are sequentially heat-treated. Since the wafers W have to be maintained at a high process temperature, the wafer boat 42 is loaded in the process chamber 8 while it is mounted on a lid 30 through an insulating cylinder 40 of quartz having a heat-insulating property.
More specifically, when the wafers W are loaded in the process chamber, they are first stacked on the wafer boat 42. Then, the wafer boat 42 is mounted on the insulating cylinder 40, and is moved up therewith into the process chamber 8 through an opening 24 by activating an elevating mechanism 26. Then, the opening 24 is airtightly closed by the lid 30.
The wafers W on the wafer boat 42 are heated and kept at a predetermined process temperature by a main heater 12 arranged around the process chamber 8. However, since the opening 24 for the wafer boat 42 should not be heated to a high temperature, the heat-insulating cylinder 40 is used. A temperature gradient is formed in the insulating cylinder 40, such that the temperature gradually decreases from the process temperature to the temperature of the opening 24.
The insulating cylinder 40 has a thermal capacity larger than the wafers W and requires a long time to be heated and become thermally stable. For this reason, the process cannot be started until the insulating cylinder 40 has a stable temperature, thereby lowering the throughput. The period of time, until the process chamber becomes thermally stable at the predetermined temperature, is called a temperature recovering time.
During the process, the insulating cylinder 40 has a stable temperature between the process temperature and the temperature of the opening 24. The insulating cylinder 40 hardly faces any part of the main heater 12, and thus is supplied with a flow of heat less than that to the wafers W. Consequently, it takes a long time for the insulating cylinder 40 and the lower wafers to become thermally stable. Further, the lower wafers are thermally affected by the insulating cylinder 40.
In order to solve these problems, several dummy wafers, which are different from wafers to be processed, may be mounted on the lower levels of the wafer boat. Even this method, however, is not sufficient to solve the problems, i.e., a long period of time up to attainment of thermal stability and a thermal ill-effect on the lower wafers.